1. Field of the Invention
This invention relates to integrated circuits and more particularly to the field of electric filters.
2. Description of the Prior Art
It has been known for a long time that certain transfer functions of filters can be performed by using passive circuits in a ladder network consisting only of inductances and capacitances. This type of circuit makes it possible to obtain very narrow passbands with a very high rejection outside the passband and relative insensitivity to variations in values of inductances and capacitances.
Unfortunately, inductors cannot be realized in accordance with integrated-circuit fabrication techniques, especially if they are intended to have high values.
Since limited space requirements essentially call for the use of fully integrated filters on very small semiconductor surfaces, filter networks requiring inductances are usually avoided in favor of purely active filters. The major drawback of such filters lies in the fact that they entail the need for a large number of operational amplifiers, namely at least n amplifiers in respect of one n-order filter. These filters are often switched-capacitor filters or in other words filters in which all the resistors have been replaced by high-frequency switched capacitors as explained in two articles published in the IEEE Journal of Solid State Circuits, volume SC-12, No 6, December 1977, pages 592 to 607 (Caves et alia, Hosticka et alia).